Thrall of Orcus is a pretty terrible prestige class, simply because it doesn't give you very much casting progression. There are very, very few abilities that a class could give you that make it worth giving up a caster level.

Advice for evil clerics? I'd look into Libris Mortis for items that help your rebuking level and reduce Turning Resistance. I think there's a rod and a lyre. Any undead you run into are suddenly your friends. Also, consider that the Corpse and Bone creature templates are much more powerful than the zombie and skeleton templates respectively, and you can apply these templates with the Create Undead spell, which might be a little costly, but will basically create full-strength minions for you.

I once made an NPC who was a Cleric of Gruumsh (horribly misspelled) that became an Evangelist out of Book of Exalted Deeds with leadership of course. He would sack a town, gather up all the towns folk and give them the options of join or die. He had an item with a variation on baleful polymorph that specifically turned someone into a generic orc. This item was a green crystal fist on an iron rod which is activated by bashing the victim on the head. If the victim survived they became an orc and were welcomed into the fold, otherwise they were dead.

As an entirely separate notion, optimization and power level isn't always important. Don't make a chump but if you want your character to be a Thrall of Orcus make it and synergy be damned. Cool is far more important than powerful